1. Field of the Invention
This invention relates to a radiation curable coating composition that when cured can easily be removed from, for instance, a glass optical fiber, especially by thermolysis. The invention further relates to a coated glass optical fiber coated with said composition in a cured state.
2. Description of Related Art
A glass optical fiber is generally coated with one or more layers of an adherent coating to protect and reinforce the glass optical fiber. The liquid coating is usually applied and cured using actinic radiation.
For some applications, it is desirable that the one or more coating layers be removed from the glass optical fiber. For example, when the ends of two glass fibers have to be connected, it is sufficient to remove the coating from a small portion of the length of the fiber. In other applications, for instance when the glass fibers are used in current sensors, it is essential that the coating be removed over the entire used length of the fiber to be employed in a given application. The use of an uncoated optical fiber in a current sensor is described in Carome, E. F. "Proceedings 1992 Workshop on optical sensing in utility applications."
It is in general a practical requirement in the production of a glass fiber to apply a coating thereto, because otherwise environmental exposure and mechanical handling will quickly reduce the fiber strength and signal transmission of the glass fiber.
One way to remove a coating from the glass fiber is to mechanically strip off the coating. Mechanical stripping has the disadvantage that pieces of the coating may remain adhered to the surface of the stripped fiber. Moreover, it is difficult to remove longer lengths of coating using mechanical stripping. It is practically impossible to remove the coating from long sections of the fiber using mechanical stripping without damaging the glass fiber.
Published German patent application DE-A4140087 discloses the removal of coating layer from the end of a glass optical fiber, especially in an optical fiber ribbon, by burning off the coating using a laser beam. The combustion products (gases and ashes) are removed from the glass fiber surface by use of a gas flow. This publication gives no description of the coating employed. However, by use of this method, a portion of the ashes will yet remain on the fiber surface.
U.S. Pat. No. 4,957,343 discloses a method for splicing a glass optical fiber in which a clad layer on the fiber is removed by pyrolysis. The amount of residue left on the glass fiber is below 10%. FIG. 7 thereof shows that about 3 weight % of residue remains on the fiber after pyrolysis.
There remains a great need for a radiation curable coating composition that in a cured state adheres to and protects the glass fiber and yet can be easily and substantially completely removed therefrom. For some applications a 10 meter, or longer, section of bare optical glass fiber is required. It is very difficult to remove such a length of coating from the optical glass fiber using conventional techniques. Thus, there is a great need for a radiation curable coating composition that in a cured state can be easily removed from such a long section of the glass optical fiber, while leaving minimal residue on the glass optical fiber.